1. Field of the Invention
The present invention relates to a beam shaping optical device, an optical head using such a device, and an optical information medium driving unit.
2. Description of the Related Art
A semiconductor laser is normally used as a light source in an optical system of an optical head for recording and regeneration of information in and from an optical disc medium such as a CD, a DVD, or a Blu-ray disc. Intensity of a light emitted from this semiconductor laser generally has an elliptical distribution. In a case of concentrating this light having the elliptical intensity distribution by use of an objective lens in the optical head, it is generally known that a spot diameter of gathered light is inversely proportional to an incident beam diameter on the objective lens, whereby the spot diameter of the gathered light becomes larger along a direction of a minor axis of the elliptical intensity distribution, than along a direction of a major axis, to reduce a resolving power of signal recording and regeneration.
In order to correct differences in the intensity distribution, a circular opening can be provided in a light path of the optical head to correct light incident on the objective lens into a circular beam. However, since part of the beam is shaded in this case, there is a disadvantage of reducing utilization efficiency of a laser beam.
Accordingly, in order to improve a light concentrating property without impairing utilization efficiency of emitted light, a beam shaping optical device for converting a flux of light having an elliptical intensity distribution, and emitted from a semiconductor laser, into a beam having an approximately circular intensity distribution has been used.
The beam shaping optical device is, for example, comprised of a collimating lens for converting a laser beam into a parallel beam, and a beam shaping prism for converting an elliptical parallel beam into a circular beam by refraction.
However, in a conventional beam shaping optical device, a member holding a lens, and an optical base on which an optical element is arranged, may undergo thermal expansion as a temperature of the beam shaping optical device changes. In such a case, parallelism of a collimated beam decreases due to displacements of a focus position of the collimating lens and a position of a light source, resulting in a problem of producing astigmatism in the collimated beam having passed the beam shaping prism. Further, a change in a refractive index of material of the collimating lens in relation to temperature, a change in a wavelength of the light source and the like may occur to cause displacements of the focus position of the collimating lens and the position of the light source. This reduces parallelism of the collimated beam, resulting in a problem of producing astigmatism in the collimated beam having passed the beam shaping prism.
A known conventional optical head includes a lens holding construction disclosed, for example, in Japanese Unexamined Patent Publication No. H10-334472. As shown in FIGS. 13A-13C, in the lens holding construction disclosed in this publication, a lens frame 124 holding a collimating lens 113 is substantially in the form of a round column and has a hollow portion having a central axis aligned with an optical axis O in order to form a light path.
One opening end of the hollow portion of the lens frame 124 is widened toward an end in order to mount a semiconductor laser. Another opening end is also widened to form an inner circumferential surface 124a in the form of a short tube. The collimating lens 113 is accommodated and held inside this inner circumferential surface 124a. A radius of the inner circumferential surface 124a is set to be slightly larger than that of an outer circumferential surface 113a of the collimating lens so that a clearance 125 is defined over an entire circumference between the inner circumferential surface 124a and the outer circumferential surface 113a in the form of a short tube.
In this holding construction, in order to position the collimating lens 113 along a direction of the optical axis, an adhesive 116 is applied to a contact surface 124b that is ring-shaped so as to be rotationally symmetric with respect to the optical axis O, and one lens surface 113b of the collimating lens 113 is brought into contact with this contact surface 124b to adhere and fix the collimating lens 113 by the adhesive 116. In this way, the collimating lens 113 is held in the lens frame 124. A radius of an outer side of the ring-shaped contact surface 124b may be extended up to the inner circumferential surface 124a of the lens frame 124. However, by making this radius smaller than a radius of the collimating lens 113 up to the outer circumferential surface 113a as shown in FIG. 13, the adhesive 116 can be prevented from coming out from between the inner circumferential surface 124a of the lens frame 124 and the outer circumferential surface 113a of the collimating lens 113. Even if the collimating lens 113 should be fixed in an eccentric manner, influence on a held state by the adhesive 116 between the lens surface 113b and the contact surface 124b can be advantageously reduced.
Further, the clearance 125 defined over the entire circumference between the inner circumferential surface 124a of the lens frame 124 and the outer circumferential surface 113a of the collimating lens 113 can provide an effect of preventing thermal deformation of the lens frame 124, due to an ambient temperature change, from directly acting on the outer circumferential surface 113a of the collimating lens 113. If the clearance 125 is provided, an effect of suppressing eccentricity of the collimating lens 113 and the lens frame 124 is weakened, but such a weakened effect is dealt with by changing an adhering method.
Furthermore, since the adhesive 116 is applied to the ring-shaped contact surface 124b provided in the lens frame 124 for positioning the collimating lens 113 with respect to a direction of the optical axis to adhere the collimating lens 113, a force trying to move the collimating lens 113 in a radial direction resulting from thermal deformation of the lens frame 124 caused by an ambient temperature change is canceled by being radially distributed. Specifically, even if the lens frame 124 thermally expands due to the ambient temperature change, radially outward forces acting on the collimating lens 113 by the thermal expansion of the lens frame 124 are canceled out by substantially equal forces acting in opposite directions on adhered portions symmetric with respect to the optical axis O since the collimating lens 113 fixed in this lens frame 124 is fixed via the adhesive 116 at a ring-shaped portion equidistant from a center of the lens frame.
A construction in which parallelism of a collimated beam does not decrease due to a change in the refractive index of the material of the collimating lens in relation to temperature and a change in the wavelength of the light source is proposed in Japanese Unexamined Patent Publication No. 2002-287018. FIG. 14 shows one exemplary construction of an optical head including a conventional beam shaping optical device disclosed in this publication. The conventional beam shaping optical device in FIG. 14 includes a light source 201, a collimating lens 202 fixed to a barrel 210, and a beam shaping optical element 203, wherein a beam having an elliptical intensity distribution and emitted from the light source 201 is converted into a parallel beam by the collimating lens 202 and further converted into a flux of light having a circular intensity distribution by the beam shaping optical element 203. The flux of light emerged from the beam shaping optical element 203 is reflected by a rising mirror 204 and focused on a recording surface of a disc 206 by an objective lens 205. This beam modulated and reflected by pit rows on the disc recording surface passes through the objective lens 205 again, is reflected by the rising mirror 204 and split by a splitting surface 203a of the beam shaping optical element 203. Thereafter, this beam is gathered by a detection lens 207 and, consequently, a modulated signal light is received by a light receiving element 208.
As disclosed in this publication, in the beam shaping optical device, a change in focal length of the collimating lens 202 is compensated by a change in a refractive index of material of the collimating lens 202 resulting from a temperature change, and a change in the refractive index of the collimating lens 202 resulting from a wavelength change caused by a temperature change of the light source, whereby a reduction in quality of a collimated beam caused by a temperature change is suppressed.
An operation in a wide temperature environment from low temperature to high temperature must be guaranteed for an optical head device. However, in the optical head shown in FIG. 13, the collimating lens 113, the lens frame 124 holding the collimating lens 113 and the lens holding construction holding the lens frame 124 undergo thermal expansion, thereby causing a relative displacement between a laser emission point and the collimating lens 113. In addition, as ambient temperature changes, a wavelength of the laser light source changes and a curvature and refractive index of the collimating lens 113 change, whereby a focal length of the collimating lens 113 deviates. As a result, there has been a problem of deteriorating collimation quality of incident light on the beam shaping optical element, i.e. parallelism of the incident light. This deterioration of the collimation quality of the incident light produces astigmatism in a flux of light after beam shaping, thereby deteriorating spot quality on the disc surface when the light is concentrated by the objective lens. As a result, there has been a problem of deteriorating recording and regeneration properties.
Further, since the collimating lens 113 has its lens surface fixed by the adhesive 116 in the conventional optical head device shown in FIG. 13, there has been a problem of varying displacements of the collimating lens 113 along the direction of the optical axis and a direction normal thereto resulting from thermal expansion and contraction of the adhesive 116 because an applied amount of the adhesive 116 differs depending on a device and depending on an adhered position. Since a resin having a high thermal expansion rate is used for the adhesive 116, an extent of expansion and contraction of the adhesive 116 resulting from a temperature change is not negligible. For example, a variation in thickness of the adhesive 116 causes a variation of an amount of change in a distance of the collimating lens 113 from the light source resulting from the temperature change. Since a width of the adhesive 116 is uneven along a circumference of the contact surface 124b, radial forces asymmetrically act on the collimating lens 113 as the temperature changes, whereby the collimating lens 113 is displaced in a direction normal to the optical axis. This displacement of the collimating lens 113 along the direction normal to the optical axis displaces the optical axis O, thereby becoming a main factor for displacing the spot of the detection light. Variation in the displacement of the collimating lens 113 resulting from variations upon applying the adhesive 116 could not be evened out.
Further, since a lens surface is adhered, there have been a problem of staining the collimating lens 113 by the adhesive 116, a problem of enlargement by providing a clearance between the lens frame 124 and the collimating lens 113, and a problem of increased costs due to a complicated shape of the lens frame 124. In addition, there has been a problem of necessitating a high adjustment precision. These problems have led to problems of deteriorated temperature characteristics and increased costs in an optical head device having the lens supporting construction integral to the barrel shown in FIG. 13, and also in an optical information device having such an optical head device.
There is also a problem that a focal length of the collimating lens changes due to the change in the wavelength of the light source caused by factors other than temperature, such as a wavelength change resulting from a change in output of the light source at a time of recording and regeneration, thereby deteriorating collimation quality of light emitted from the beam shaping optical device.
Since refractive indices of general optical materials largely change at wavelengths in a short wavelength region, deterioration of the collimation quality of light emitted from the beam shaping optical device is conspicuously seen if a light source having a short wavelength is used such as in the case of a Blu-ray disc.